Optical system and projection-type image display device

ABSTRACT

In an optical system includes a first prism having a first surface and a second surface forming mutually acute angles, and a second prism having a third surface. The first prism and the second prism forms an air gap between the second surface and the third surface. The air gap formed by the second surface and the third surface is a cross section perpendicular to the first surface and the second surface having a wedge shape which widens at a part near the first surface and narrows at a part distant from the first surface.

RELATED APPLICATION

[0001] This application is based on Patent Application No.2000-236430 filed in Japan, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical system comprising a plurality of prisms, and a projection-type image display device provided with this optical system.

[0004] 2. Description of the Related Art

[0005] There are projection-type image display devices which direct illumination light to a reflection-type display element, modulate the illumination light by projection image displayed on the display element, and display a projection image on a screen by projecting the reflected modulation light through a projection optical system. Reflection-type liquid crystal displays (LCD), or digital micro mirror devices™ (DMD™, both DMD™ and digital micro mirror devices™ are trademarks of Texas Instruments) are used as the display element.

[0006] A reflection-type LCD modulates illumination light entering from an approximately perpendicular direction via a liquid crystal layer displaying the projection image, and reflects the image in an approximately perpendicular direction. A DMD™ has micro mirror elements of variable directionality arranged in a plurality of rows on a plane. The direction of each mirror element alternatively selectable from among a specified two directions, which are selected in accordance with the displayed image. Illumination light comprises light of a displayed projection image reflected in one direction modulated light of an undisplayed projection image reflected in another direction. The range of variance of direction of the mirror elements is very slight at the micro level, and the DMD™ receives illumination light from a near-perpendicular direction, and reflects light in a second near-perpendicular direction.

[0007] In this way illumination light of modulated light from one direction must be directed to the display element in a projection-type image display device which modulates illumination light by a reflection-type display element. Accordingly, an optical system for directing illumination light to the display element without blocking the modulated light must be arranged medially to the display element and the projection optical system, and an optical system comprising a plurality of prisms is often used for this purpose.

[0008] An example of a conventional optical system is shown in FIG. 7. Parts (a) and (b) of FIG. 7 are mutually intersecting cross section views of an optical system 50. The optical system 50 comprises two prisms 51 and 52. Prism 51 has three surfaces 51 a, 51 b, 51 c, and surfaces 51 a and 51 bform an acute angle. Prism 52 also has three surfaces 52 a, 52 b, 52 c, and surfaces 52 a and 52 b form an acute angle.

[0009] Prisms 51 and 52 are arranged such that surface 51 b confronts surface 52 b with an interval of a small distance therebetween. That is, a small air gap G is formed between surface 51 b and surface 52 b. Surface 51 b and surface 52 b are mutually parallel, and the size (thickness) of the air gap G is constant regardless of position.

[0010] The angle formed by the surfaces 51 a and 51 b of the prism 51 and the angle formed by the surfaces 52 a and 52 b of the prism 52 are equal, and, accordingly, the surfaces 51 a and 52 a are parallel. an optional axis perpendicular to the surface 51 a of prism 51 is referred to as optical axis Ax of the optical system 50. A display element is arranged perpendicular to the optical axis Ax on the surface 51 a side, and a projection optical system is arranged such that the optical axis of the projection optical system is parallel to the optical axis Ax on the surface 52 a side. Accordingly, the air gap G, and the surfaces 51 b and 52 b forming this air gap G are oblique to the optical axis of the projection optical system.

[0011] The direction of the optical axis Ax is referred to as the X direction, the direction perpendicular to the optical axis Ax within a plane perpendicular to the air gap G is referred to as the Y direction, and the direction perpendicular to the optical axis Ax within a plane parallel to the air gap G is referred to as the Z direction. FIG. 7(a) is the cross section in the X-Y plane, and FIG. 7(b) is the cross section in the X-Z plane.

[0012] The light for illuminating the display element passes through the surface 51 c of the prism 51 in the optical system 50. The light passing through the surface 51 c and entering the prism 51 and reaches the surface 51 b. The incidence angle of light on the surface 51 b is set so as to exceed the critical angle, and the light is completely reflected by the surface 51 b. The light completely reflected by the surface 51 b reaches the surface 51 a, is transmitted through the surface 51 a, and impinges the display element at an approximate perpendicular.

[0013] Light impinging the display element is modulated and reflected by the projection image displayed on the display element. The modulated reflected light is transmitted through the surface 51 a, and impinges prism 51, reaching the surface 51 b. The entrance angle of this light on the surface 51 b is less than the critical angle, and the light is transmitted through the surface 51 b, crosses the air gap G, and impinges the prism 52 from the surface 52 b. The light impinging the prism 52 reaches the surface 52 a, is transmitted therethrough, impinges the projection optical system, is projected therefrom and forms a projection image on the screen.

[0014] The modulated light is refracted when transmitted through the surfaces 51 b and 52 b, however, since the surfaces 51 b and 52 b are parallel, the optical path is also parallel before passing through surfaces 51 b and 52 b, and after passing through surfaces 51 b and 52 b. Since the air gap G is oblique to the optical axis Ax, the size of the shift in the optical path before transmission and after transmission is different in the mutually perpendicular Y direction and Z direction. Therefore, although the light has the same point of origin, the origin point in the Y direction is positioned nearer to the projection optical system than the origin point in the Z direction. The shift of the Y direction and Z direction origin points in the optical axis Ax direction is referred to as the interval difference. The origin point in the Z direction is one point, however, the origin point in the Y direction is broadened.

[0015] Since the shift of these origin points causes distortion in the image formed by the projected light and reduces the quality of the displayed projection image, this shift must be suppressed as much as possible. For this reason, the size of the air gap is very small, approximately 10 μm in a conventional optical system. FIG. 8 shows the relationship between the amount of defocus and the optical transfer function (OTF) when the size of the air gap G is set to this degree. In FIG. 8, the curves marked by the symbols XY and XZ represent the OTF within the XY plane and the XZ plane, respectively. Both the amount of defocus of the horizontal axis or the OTF of the vertical axis is standardized when there is no air gap within the optical system. In FIG. 8, there is no great difference in the OTF of the XY plane and the OTF of the XZ plane, and excellent image forming performance is obtained.

[0016] When a color projection image is provided, light from a light source emitting white light is split into red (R), green (G), and blue (B) light, and each color light after splitting is modulated by separate reflection-type display elements. In this case an optical system having two air gaps of constant size are used, an a dichroic film is provided on one surface forming each air gap, and, for example, red light is reflected and green light and blue light are transmitted by one dichroic film, and, for example, blue light is reflected and red light and green light are transmitted by the other dichroic film so as to split the red light, green light, and blue light.

[0017] Three display elements are provided to display the R component, G component, and B component of the projection image. The R light and B light reflected by the dichroic film are completely reflected by different surfaces of the prism, and directed to the corresponding display element. The light modulated and reflected by each display element follows the optical path in reverse and is combined within the optical system, and projected by the projection optical system. In this case also, the size of the air gap is approximately 10 μm, such that an excellent color image is displayed without color shift.

[0018] In recent years, extremely high intensity light has been used as illumination light in optical system in accordance with demand for high luminance projection images. Although each prism of the optical system is manufactured using material of high transmittance, light energy is absorbed and a high temperature is reached, and swelling cannot be avoided. This swelling deforms the surfaces of the prism, and reduces the performance of the prism.

[0019] The deformation of the surface opposite the display element and the projection optical system is only somewhat connected to performance reduction, however, when the surfaces forming the air gap are deformed, both surfaces come into contact, and the parts in contact cannot produce complete reflection. When complete reflection cannot be attained, the illumination light cannot be directed to the display element and optical system does not function.

[0020] In a projection-type image display device which displays a bright projection image, the air gap within the optical system is made large so as to prevent contact between the surfaces of the two prisms forming the air gap even when the prisms swell. FIG. 9 shows an optical system 50′ in which the air gap is large. in this optical system 50′, the air gap G of the optical system 50 of FIG. 7 is enlarged from approximately 10 μm to approximately 50 μm. When the air gap becomes this large, the shift of the origin point of the light also increases, and the interval difference becomes extremely large.

[0021]FIG. 10 shows the relationship between defocus and OTF in the optical system 50′. There is a shift of approximately 0.05 mm in the maximum position of the OTF in the XY plane and the XZ plane, and the quality of the image displayed on the screen is greatly reduced.

[0022] Disadvantages accompanying an increase in the air gap in an optical system have long been known, however, simple and effective countermeasures have yet to be proposed. Although it is possible to suppress distortion by using an anamorphic projection optical system, an increase in cost is unavoidable, it becomes necessary to design the projection optical system in accordance with each individual optical system, and interchangeability is lost.

SUMMARY OF THE INVENTION

[0023] An object of the present invention is to provide an improved optical system and a projection-type image display device provided with this optical system capable of providing a bright, high-quality projection image.

[0024] Another object of the present invention is to provide an optical system capable of suppressing the difference in the path of light in two directions, and a projection-type image display device provided with this optical system and capable of providing a bright, high-quality image.

[0025] These objects are attained by the present invention, in an optical system including a first prism having a first surface and a second surface forming mutually acute angles, and a second prism having a third surface, and forming an air gap between the second surface and the third surface, wherein an air gap formed by the second surface and the third surface is a cross section perpendicular to the first surface and the second surface having a wedge shape which widens at a part near the first surface and narrows at a part distant from the first surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] These and other objects and features of this invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings, in which:

[0027]FIG. 1 is a cross section view schematically showing the structure of an optical system of a first embodiment;

[0028]FIG. 2 shows the relationship between the amount of defocus and OTF of the optical system of the first embodiment;

[0029]FIG. 3 is a cross section view of an example of a structure having a wedge-shaped cross section of the air gap in the optical system of the first embodiment;

[0030]FIG. 4 is another cross section view of an example of a structure having a wedge-shaped cross section of the air gap in the optical system of the first embodiment;

[0031]FIG. 5 is a cross section view schematically showing the structure of the projection-type image display device of a second embodiment;

[0032]FIG. 6 is a top view and side view of another example of a structure of an optical system of the essential part of the projection-type image display device of a third embodiment;

[0033]FIG. 7 is a cross section view schematically showing the structure of a conventional optical system;

[0034]FIG. 8 shows the relationship n=between the amount of defocus and OTF in the optical system of FIG. 7;

[0035]FIG. 9 is a cross section view schematically showing the structure of a conventional optical system having a large air gap; and

[0036]FIG. 10 shows the relationship between the amount of defocus and OTF in the optical system of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The embodiments of the optical system and projection-type image display device of the present invention are described hereinafter with reference to the accompanying drawings. The structure of the optical system of a first embodiment is schematically shown in FIG. 1. In FIG. 1, (a) and (b) are mutually intersecting cross section views of an optical system 1.

[0038] The optical system 1 comprises two prisms 11 and 12. Prism 11 has three surfaces 11 a, 11 b, 11 c, and surfaces 11 a and 11 b form an acute angle. Prism 12 has three surfaces 12 a, 12 b, 12 c, and surfaces 12 a and 12 b form an acute angle.

[0039] Prisms 11 and 12 are arranged such that surface 11 b confronts surface 12 b separately by a small distance. That is, surfaces 11 b and 12 b form an air gap G. Surface 11 b and surface 12 b are not parallel, and are set so as to have a relatively large distance at a part near the surface 11 a, and a relatively small distance at a part far from the surface 11 a. Accordingly, the cross section of the air gap G is wedge-shaped having a width separated from the surface 11 a.

[0040] The surface 11 a of the prism 11 and the surface 12 a of the prism 12 are approximately parallel. An optional axis perpendicular to the surface 11 a of the prism 11 is referred to as the optical axis Ax of the optical system 1, the direction of the optical axis Ax is designated the X direction, and a direction perpendicular to the optical axis Ax within a plane perpendicular to the air gap G is designated the Y direction, and a direction perpendicular to the direction of the optical axis Ax within a plane parallel to the air gap G is designated the Z direction. FIG. 1 (a) is a cross section in the X-Y plane, and (b) is a cross section in the X-Z plane.

[0041] The optical paths before and after light from an object point positioned on the surface 11 a side passes through the air gap G are parallel within the X-Z plane and are not parallel in the X-Y plane due to the wedge shape of the air gap G. Moreover, since the size (thickness) of the air gap G increases at a part near the surface 11 a and decreases at a part far from the surface 11 a, the width of the flux in the Y direction impinging the surface 12 b after crossing the air gap G is approximately identical to the width in the Z direction. For this reason there is no difference in width in the Y direction and Z direction, and widening of the origin point of the light in the Y direction is eliminated. Accordingly, even after passing through the prisms 11 and 12, there is no difference produced in the Y direction and Z direction in the course of the light from the origin point positioned on the surface 11 a side.

[0042] The relationship between the amount of defocus and OTF in the optical system 1 is shown in FIG. 2. The degree of matching of the OTF in the X-Y plane and the X-Z plane is high, and there is no apparent influence of the inclination of the air gap G relative to the optical axis Ax.

[0043] The degree of the wedge of the cross section of the air gap G, i.e., the size of the angle formed by the surface 11 b and surface 12 b in a plane perpendicular to the surface 11 b and surface 11 a, is determined in accordance with the size of the air gap G and the distance from the object point positioned on the surface 11 a side to the surface 11 a. When a general application wherein light from the display element is directed to a projection optical system for projection through an optical system, the angle formed by the surface 11 b and the surface 12 b may be 0.25° or less. In FIG. 1, the angle formed by the surface 11 b and the surface 12 b is shown large to clarify the wedge shape of the air gap G.

[0044] Since the air gap G is wedge-shaped, the optical axis Ax perpendicular to the surface 11 a of the prism 1 is not strictly perpendicular to the surface 11 a within prism 12. When the surface 12 a if the prism 12 is strictly parallel to the surface 11 a, the optical axis Ax on the exterior side of the surface 12 a is not strictly perpendicular to the surface 11 a. In the optical system 1, the degree of the wedge shape of the air gap G is considered when determining the angle formed by the surface 11 a and surface 12 a such that the optical axis Ax even on the exterior side of the surface 12 a is perpendicular to the surface 11 a. Actually the surface 12 a can be said to be parallel to the surface 11 a since the angle formed by the surface 11 a and the surface 12 a is 0.25° or less.

[0045] An example of a structure providing the air gap G with a wedge-shaped cross section is shown in FIG. 3. In this case two glass plates 13 a and 13 b are installed medially to the prisms 11 and 12, and these glass plates 13 a and 13 b are sandwiched between the surfaces 11 a and 11 b. The glass plate 13 a at a part near the surface 11 a is thicker than the glass plate 13 b at a part far from the surface 11 a, such that the cross section of the air gap G has a wedge shape. The degree of the wedge shape can be set freely by the degree of thickness of the glass plates 13 a and 13 b, and may be finely adjusted by changing the distance between the glass plates 13 a and 13 b.

[0046] Another example of a structure of a wedge-shaped cross section of the air gap G is shown in FIG. 4. In this case glass beads 14 a and 14 b are adhered to the prisms 11 and 12 by adhesive 14 containing the glass beads, such that the glass beads 14 a and 14 b are sandwiched between the surfaces 11 b and 12 b. The glass bead 14 a at a part near the surface 11 a is larger than the glass bead 14 b at a part far from the surface 11 a, and in this way the cross section of the air gap G is wedge-shaped. In this structure, the prisms 11 and 12 are fixed at the same time the air gap G is formed, which makes the assembly of the optical system 1 extremely easy.

[0047] Plate-like member or particles formed of other materials may be used in place of the glass plates 13 a and 13 b and glass beads 14 a and 14 b insofar as the materials have small thermal expansion coefficient and excellent thermal resistance. For example, ceramic plates and ceramic particles may be used.

[0048] The construction of a projection-type image display device 2 of a second embodiment is schematically shown in FIG. 5. The projection-type image display device 2 comprises a lamp 21, reflector 22, integrator rod 23, relay lenses 24 and 26, plane mirror 25, optical system 1 of the first embodiment, display element DMD™ 27, and projection optical system 28.

[0049] The lamp 21 emits white light to illuminate the DMD™ 27, and the reflector 22 converges the emitted light which impinges the integrator rod 23. The reflector 22 is a rotating hyperbolic mirror, and lamp 21 is disposed at a first focus. The entrance end of the integrator rod 23 is disposed at a second focus of the reflector 22, and light emitted from the lamp 21 is converged at the entrance end to the integrator rod 23.

[0050] The light entering the integrator rod 23 is completely reflected by the side surfaces and reaches the exit end. The frequency of total reflection by the side surfaces of the integrator rod 23 is dependent on the incidence angle of the light entering the entrance end. Accordingly, the center of the flux from the reflector 22 and peripheral light are mixed at the exit end, such that there is uniform intensity distribution of the light exiting from the integrator rod 23.

[0051] The relay lenses 24 and 26 direct the light emitted from the integrator rod 23 to the whole surface of the DMD™ 27, and the mirror 25 reflects the light between the relay lenses 24 and 26 so as to bend the optical path. The exit end of the integrator rod 23 and the DMD™ 27 have a conjugate positional relationship, such that the DMD™ 27 is uniformly illuminated. The mirror 25 is provided to reduce the size of the device, and may be omitted.

[0052] Light from the relay lens 26 enters the optical system 1 from the surface 11 c. Light passing through surface 11 c and entering the prism 11 reaches the surface 11 b. The incidence angle of the light on the surface 11 b is set so as to exceed the critical angle, and the light is completely reflected by the surface 11 b. The light completely reflected by the surface 11 b is directed to the surface 11 a, passes through the surface 11 a, and impinges the DMD™ 27 from a somewhat inclined direction.

[0053] The DMD™ 27 is set such that the light representing a projection image among the reflected light advances perpendicular to the surface 11 a of the prism 11, i.e., parallel to the optical axis of the optical system 1. The DMD™ 27 is further set such that the light (not shown) not representing a projection image among the reflected light advances in a direction (up direction in FIG. 5) of the opposite side to the illumination light relative to the optical axis of the optical system 1.

[0054] Light representing the projection image from the DMD™ 27 passes through the surface 11 a, enters the prism 11, and reaches the surface 11 b. The incidence angle of the light reaching the surface 11 b is less than the critical angle, such that the light passes through he surface 11 b, crosses the air gap G, and enters the prism 12 from the surface 12 b. The light entering the prism 12 reaches the surface 12 a, passes therethrough, and enters the projection optical system, 28.

[0055] The projection optical system 28 projects light from the optical system 1 toward a screen (not illustrated), and forms and image on the screen to display a projection image. The light not representing the projection image from the DMD™ 27 also passes through the optical system 1, however, this light does not enter the projection optical system 28.

[0056] When a color projection image is displayed by the projection-type image display device 2, color wheel provided with color filters for selectively transmitting R light, G light, and B light is disposed near the entrance end or the exit end of the integrator rod 23, so that the light illuminating the DMD™ 27 can be switched. Furthermore, the DMD™ 27 may display an image corresponding to the illumination light among the R component, G component, B component of the projection image at the same time as the illumination light is switched.

[0057] The structure of an optical system 3 forming the essential part of a projection-type image display system of a third embodiment is schematically shown in FIG. 6. Parts (a) and (b) of FIG. 6 respectively show a top view and side view of the optical system 3. The optical system 3 comprises five prisms 31, 32, 33, 34, 35. Prisms 31 and 32 are identical to prisms 11 and 12 of optical system 1 of the first embodiment, and surfaces 31 b and 32 b form a small air gap G having a wedge-shaped cross section between surfaces 31 b and 32 b.

[0058] Prism 33 has surfaces 33 a and 33 b which mutually form an acute angle. Surface 33 a is parallel to the surface 31 a of the prism 31. Prism 34 has surfaces 34 a and 34 b which also mutually form an acute angle, and, similarly, prism 35 has surfaces 35 a and 35 b which also mutually form an acute angle. The surface 35 a of the prism 35 is approximately parallel to the surface 31 a of prism 31.

[0059] Prisms 33, 34 and 35 are arranged so as to form a small air gap G1 having a wedge-shaped cross section between surfaces 33 b and 34 a, and form a small air gap G2 having a wedge-shaped cross section between the surfaces 34 b and 35 b. The plane perpendicular to the air gaps G1 and G2 intersect a plane perpendicular to the air gap G. Prism 31 and prism 35 are adjacent, and surfaces 31 a and 35 a form a small air gap G3 of approximately uniform size.

[0060] The surface 34 a of prism 34 is provided with a dichroic film 36B for reflecting B light and transmitting light of other wavelengths. Furthermore, surface 35 b of the prism 35 is provided with a dichroic film 36R for reflecting R light and transmitting light of other wavelengths.

[0061] The projection-type image display device of the present embodiment substitutes the optical system 3 for the optical system 1 of the projection-type image display device 2 described above, and provides three display elements in place of a single display element so as to display color projection images. Other aspects are identical to the projection-type image display device 2, and their repeated description is omitted.

[0062] Although not shown in the drawing, the three display elements are respectively arranged opposite the surface 33 a of the prism 33, the surface 34 c of the prism 34, and the surface 35 c of the prism 35. A DMD™ 27 or reflective type LCD is used as the these display elements. The display element opposite the surface 33 a displays the G component, the display element opposite the surface 34 c displays the B component, and the display element opposite the surface 35 c displays the R component, and the light can be modulated by the displayed projection image. Furthermore, the projection optical system is arranged such that the optical axis of the projection optical system is parallel to the optical axis Ax of the optical system 3 on the surface 32 a side of the prism 32.

[0063] The light for illuminating the display elements is provided to the optical system 3 from the surface 31 c of the prism 31. Light entering prism 31 from the surface 31 c is completely reflected by the surface 31 b, passes through the surface 31 a, crosses the air gap G3, and enters the prism 35 from surface 35 a. this light reaches the surface 35 b, and the R light contained therein is reflected by the dichroic film 36R, and the G light and B light are transmitted through the dichroic film 36R.

[0064] The reflected R light impinges the surface 35 a at an incidence angle exceeding the critical angle, and is completely reflected. This R light passes through the surface 35 c, and illuminates the display element for displaying the R component of the projection image, and this light is modulated, and reflected perpendicularly.

[0065] The G light and B light transmitted through the dichroic film 36R crosses the air gap G2, enters the prism 34 from surface 34 b, and reaches the surface 34 a. The B light is reflected by the dichroic film 36B, and the G light is transmitted through the dichroic film 36B.

[0066] The reflected B light enters the surface 34 b at an incidence angle exceeding the critical angle, and is completely reflected. The B light passes through the surface 34 c, and illuminates the display element for displaying the B component of the projection image, and is modulated and reflected perpendicularly.

[0067] The G light transmitted through the dichroic film 36B crosses the air Gap G1 and enters the prism 33 from the surface 33 b. This G light passes through the surface 33 a, and illuminates the display element for displaying the G component of the projection image, and is modulated and reflected perpendicularly.

[0068] The R light, G light, and B light reversely return through the optical path, and enter the prism 31 from the surface 31 a. During this time the G light and B light are combined by the dichroic film 36B, and the red light and the combined G light and B light are combined by the dichroic film 36R.

[0069] The light representing the projection image entering the prism 31 passes through the surface 31 b, crosses the air gap G, and passes through the prism 32, and thereafter enters the projection optical system, and a color projection image is displayed on the screen.

[0070] Although air gaps G1 and G2 are formed to provide dichroic films 36R and 36B in the optical system 3, these air gaps G1 and G2 have a wedge shape which is narrower at a part near the projection optical system identical to the air gap G, such that no difference is generated in the course of the light from the display elements across the air gaps G1 and G2 in two intersecting directions. Accordingly, the displayed color image is high quality without color shift.

[0071] In the optical system of the present invention wherein the air gap between prisms has a wedge shape, the light from an object point at a position on the first surface side is transmitted and does not have a difference in course in two intersecting direction even when the air gap is increased. Accordingly, the optical system is suitable for high intensity light.

[0072] The shape of the cross section of the air gap can easily be set by interposing plate-like members of different thickness between the first surface and the second surface at different parts, or adhering adhesive containing particles of different size at different parts of the second surface and the third surface.

[0073] generation of a difference in the course of light transmitted through the optical system in two intersecting directions is adequately suppressing a practical range of distance between a first surface and an object point at a position on the first surface side by setting the angle formed by the second surface and the third surface at 0.25° or less.

[0074] When the second prism has a fourth surface approximately parallel to the first surface, light from the object point on the first surface side can be transmitted without causing angular change in the principal ray, providing an optical system of even simpler structure.

[0075] When the structure includes a third prism having a surface approximately parallel to the first surface, and an air gap having a wedge-shaped cross section is formed between the second prism and the third prism, light from an object point on the first surface side can be transmitted without angular change in the course of the principal ray, and the use of two wedge-shaped air gaps improved performance.

[0076] When a dichroic film is provided on the farthest surface from a first surface among the two surfaces forming each air gap, light enters the third prism from a surface approximately parallel to the first surface, and this light can be split into light of three wavelength ranges. Moreover, if the split light is modulated and reflected perpendicularly, each modulated light can be combined and emitted from the third prism.

[0077] Advantages of the projection-type image display device of the present invention include the aspect of difficulty in generating distortion in the projected light, and an extremely bright and sharp projection image even when high intensity light is used to illuminate the display element.

[0078] A color image without color shift is produced by an optical system having three display elements, and dichroic film.

[0079] Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. 

What is claimed is:
 1. An optical system comprising: a first prism having a first surface and a second surface forming mutually acute angles; a second prism having a third surface, and forming an air gap between the second surface and the third surface; wherein an air gap formed by the second surface and the third surface is a cross section perpendicular to the first surface and the second surface having a wedge shape which widens at a part near the first surface and narrows at a part distant from the first surface.
 2. The optical system as claimed in claim 1, wherein the second surface and the third surface form an air gap by sandwiching plate-shaped members of different thickness at a part near the first surface and a part distance from the first surface.
 3. The optical system as claimed in claim 1, wherein the second surface and the third surface form an air gap by sandwiching particles of different size included in an adhesive at a position near the first surface and at a position distant from the first surface.
 4. The optical system as claimed in claim 1, wherein the angle formed by the second surface and the third surface is 0.25° or less within a plane perpendicular to the first surface and the second surface.
 5. The optical system as claimed in claim 1, wherein the second prism has a fourth surface approximately parallel to the first surface.
 6. The optical system as claimed in claim 1, wherein a third prism is included, the second prism has a fourth surface forming an acute angle with the third surface, and the third prism has a fifth surface forming an air gap between the fourth surface, and a sixth surface approximately parallel to the first surface, and an air gap formed by the fourth surface and fifth surface has a cross section perpendicular to the third surface and the fourth surface and is wedge-shaped so as to widen at a part near the third surface and narrow at a part distant from the third surface.
 7. The optical system as claimed in claim 6, wherein the third surface is provided with a first dichroic film to reflect light of a first wavelength range and transmit light other than the first wavelength range, and the fifth surface is provided with a second dichroic film to reflect light of a second wavelength range and transmit light other than the second wavelength range.
 8. A projection-type image display device comprising: a display element for displaying a projection image, and modulating and reflecting illumination light by the displayed projection image; a projection optical system for projecting light modulated by the display element; an optical system disposed medially to the display element and the projection optical system for directing illumination light to the display element, and directing light modulated by the display element to the projection optical system; said optical system comprising: a first prism having a first surface and a second surface forming mutually acute angles; a second prism having a third surface, and forming an air gap between the second surface and the third surface; wherein an air gap formed by the second surface and the third surface is a cross section perpendicular to the first surface and the second surface having a wedge shape which widens at a part near the first surface and narrows at a part distant from the first surface; and wherein the optical system is arranged such that the first surface forms a right angle to the optical axis of the projection optical system, and the second surface is positioned nearer the projection optical system than the first surface.
 9. The projection-type image display device as claimed in claim 8, wherein the second surface and the third surface of the optical system for an air gap by sandwiching plate-like members of different thickness at a position near the first surface and at a position distant from the first surface.
 10. The projection-type image display device as claimed in claim 8, wherein the second surface and the third surface of the optical system form an air gap by sandwiching particles of different size included in an adhesive at a position near the first surface and at a position distant from the first surface.
 11. The projection-type image display device as claimed in claim 8, wherein the angle formed by the second surface and the third surface is 0.25° or less within a plane perpendicular to the first surface and the second surface of the optical system.
 12. The projection-type image display device as claimed in claim 8, wherein the second prism of the optical system has a fourth surface approximately parallel to the first surface.
 13. The projection-type image display device as claimed in claim 8, wherein illumination light is directed to the display element via total reflection by the second surface of the optical system.
 14. A projection-type image display device comprising: three display elements for respectively displaying projection images of different wavelength, and modulating and reflecting illumination light by the displayed projection images; projection optical system for projecting light modulated by the three display elements; an optical system disposed medially to the three display elements and the projection optical system for splitting illumination light according to wavelength and directing illumination light to the three display elements, and combining light modulated by the three display elements and directing it to the projection optical system; said optical system comprising: a first prism having a first surface and a second surface forming mutually acute angles; a second prism having a third surface, and forming an air gap between the second surface and the third surface; a third prism wherein the second prism has a fourth surface forming an acute angle with the third surface, and the third prism has a fifth surface forming an air gap between the fourth surface, and a sixth surface approximately parallel to the first surface, and an air gap formed by the fourth surface and fifth surface has a cross section perpendicular to the third surface and the fourth surface and is wedge-shaped so as to widen at a part near the third surface and narrow at a part distant from the third surface; wherein the optical system is arranged such that the first surface forms a right angle to the optical axis of the projection optical system, and the second surface is positioned nearer the projection optical system than the first surface; and wherein the third surface is provided with a first dichroic film to reflect light of a first wavelength range and transmit light other than the first wavelength range, and the fifth surface is provided with a second dichroic film to reflect light of a second wavelength range and transmit light other than the second wavelength range. 